Abstract : Metal-atom-doped solid hydrogen has shown promise as a High Energy Density Material with greatly improved propulsion properties; however, the production of high concentrations of atoms in a cryogenic medium has been highly problematic. The objective of this contract was to investigate at a fundamental level the intermolecular forces which determine the structure and properties of the dopant atoms in the host matrix. Aluminum atoms in argon clusters provided a model system with which to test theoretical methods being developed at Phillips Laboratory, Edwards AFB. Spectroscopic measurements of these clusters were performed on a new spectrometer developed under the parent contract. Comparisons of the observations with theoretical calculations revealed the inadequacies of many common assumptions and demonstrated the importance of the mixing of electronic states of the atoms in the presence of the perturbing cluster or matrix. These results provided a benchmark for theory and yield important insights into the interpretation of the electronic spectra of doped matrices.